1. Field of the Invention
The present invention relates to a device for controlling the drive of a power window. The device makes it possible to control a means both for maintaining the state of a switch operated for raising or lowering a door glass by a driving source such as a motor and for continuing the raising or lowering of the door glass.
2. Description of the Related Art
In a power window apparatus, a motor is usually used for raising and lowering a door glass. When a switch provided in a vehicle compartment (for example, on a door trim) is operated, the motor is driven until the door glass is completely closed or is completely opened.
The switch is designed for two-step operation. When a passenger releases the switch after holding the switch at a position for the first step, the switch is automatically turned off. When the passenger moves the switch to a position for the second step, an iron core having a coil produces magnetic force so that the switch is held in the second step by the magnetic force (magnet type switch). Accordingly, even after the passenger releases the switch, the door glass continues its upward or downward movement until the door glass is completely closed or is completely opened.
In a conventional power window apparatus, a motor current is detected using a resistor having a very small resistance (a shunt resistor) to stop the generation of magnetic force from the iron core (i.e., to shut off the current supplied to the coil) when the door glass is brought into a locked state, which occurs when the door glass is completely closed or is completely opened. As a result, the switch is returned to its off-position by an urging means.
When the switch cannot be returned by the urging means due to mechanical trouble, the supply of power to the motor is continued so that the motor abnormally heats up, which becomes a cause of breakdown. Accordingly, the conventional apparatus is provided with not only a mechanism for shutting off the supply of electricity by stopping the magnetic force but also a bimetallic switch interposed in a power supplying line connected to the motor. When heat is abnormally generated, the supply of electricity is shut off by the bimetallic switch.
However, since the mechanism for shutting off the electricity supplied to the motor must be independently provided for canceling the automatic operation and for preventing the motor from abnormally heating, the number of parts increases, resulting in decreased efficiency of assembly work. Also, since the bimetallic switch has poor reliability, periodic checks are needed, thereby decreasing the efficiency of maintenance work.